The focus of this book is to prepare for an exam that covers supporting and
troubleshooting Cisco networks. In order to prepare for this exam, you need
to understand some of the principles and media types used in today's networks
in order to begin troubleshooting. You should have a good knowledge of different
topologies and how certain media standards operate. The IEEE media standards
you should know for the exam are the Ethernet, Fast Ethernet, Gigabit Ethernet,
ATM, Token Ring, and wireless standards.

For the exam, Cisco expects you to know many aspects of the network. You should
understand how hubs, bridges, switches, and routers segment the networks, as
well as how they divide broadcast and collision domains. In this chapter, I
give you an overview of the different LAN topologies and the IEEE LAN media
standards you need to know in order to do well on the CCNP exam.

LAN Topology Types

In the next three sections I will explain the three different network
topologies you need to know for the exam. A network topology is all the combined
physical, logical, or virtual components that make up the network or network
segment. Although there are many topologies, we will focus on those necessary
for the exam, which include the bus, ring, and star topologies.

Bus

A bus topology is a local area network (LAN) where each of the networked devices
are attached to a single cable or link, as shown in Figure
3.1. In a bus topology, stations are attached to a linear multiport medium
where only half-duplex operations exist between a station and a bus.

NOTE

Half-duplex is where communication occurs bi-directionally on one cable. This
means that a device sending data cannot receive data at the same time.
Full-duplex uses two individual cables, one to send and another to receive. This
allows a device to send at the same time it receives data.

Frames that are transmitted to the bus provide the address of the
frame's destination. If the frame gets to the end of the link and the frame
has not found its intended destination, then the frame is lost. In a bus
topology, there is no security; every node attached to the line can see the
conversations of the other nodes on the link.

Figure
3.1 A bus topology LAN. Notice that all the workstations are connected
by a single cable.

Ring

In a ring topology LAN (shown in Figure
3.2), as in a bus topology LAN, all the nodes or devices in the network
are attached to the network on the same cable or link. The difference is that
a ring topology makes a complete circle. Both Token Ring/IEEE and Fiber Distributed
Data Interface (FDDI) use a ring topology. FDDI is an American National Standard
Institute (ANSI) X3T9.5 standard cable, which now supports up to Gigabit speeds
using fiber-optic cabling. It can use a single ring for half-duplex operations
or a dualring architecture for full-duplex operations.

Figure
3.2 A ring topology LAN. Notice that all of the nodes connect to the ring.
Data for most implementations travels in one direction on the ring. However, many
technologies, including Token Ring, allow for a second ring which allows for full-duplex
operations.

When a break in the ring occurs, such as a cut cable or other cabling
problem, it affects all the stations. This means that none of the stations
connected can receive or transmit data. The longer the cable or link and the
more attached stations, the more repeaters that are needed. However, due to
timing distortions within signals, a limited number of repeaters can be used in
the same network. In a ring-topology network, centralized access means that
faults are easy to detect and isolate. Multiple rings are sometimes used to make
a very robust and reliable network.

Star

The star topology is the most common topology in today's networks, and
includes Ethernet, Fast Ethernet, and Gigabit Ethernet. Each node in a star
topology connects to a dedicated link where the other end connects to a switch
or hub. In the star-topology network shown in Figure
3.3, multiple devices are connected to a switch or hub.

Figure
3.3 A Star topology LAN. Notice that each workstation is directly connected
to a hub or switch.

One of the best reasons to use a star topology is that a loss of any node
will not disrupt network operations. It is also easy to add or remove a node
from the network. From wiring to installation, it is particularly easy to set up
a star topology network.